A disc stack divides the bowl into thin channels, so a fat globule has to cross less than a millimeter of milk instead of tens of millimeters, which cuts the separation time by tens of times at the same g-force. With 30 discs spaced 0.75 mm apart, the idealized settling area is about a thousand square meters, enough in theory for several hundred to several thousand liters an hour, which means a 50-100 liters an hour hand separator is limited by power and gap accuracy rather than settling area.
Part of Centrifuge Building
The disc stack is a set of closely spaced cones inside the bowl that divide the milk into thin sheets, so that the cream and skim separate across a fraction of a millimeter rather than across the whole bowl.
Thin Layers Replace A Long Travel
Without discs, a fat globule near the bowl wall must travel inward through the whole liquid layer. In the reference bowl (150 mm diameter, 8,000 RPM, 5,366 g at the rim), the cream travels about 55 mm from the rim to the cream zone 20 mm from the axis. At the globule speed of 3.6 mm/s computed in Separation Theory, that takes 55 ÷ 3.6 = 15 seconds. The inputs for that speed (a 4 µm globule, 1.5 mPa·s milk) are standard dairy values from general knowledge, not wiki figures.
A disc divides the liquid into sheets only as thick as the gap. The globule now has to cross the gap rather than the whole bowl.
Worked Example 1: The Distance Saved
Discs are cones, so the centrifugal acceleration is not perpendicular to them. If the cone makes an angle β with the axis, the part of the acceleration that pushes a globule across the gap is a × cos β. Take β = 40° as an assumption (the angle is a design choice and the carrier does not give one). cos 40° = 0.77.
- Speed across the gap: 3.6 × 0.77 = 2.8 mm/s.
- Gap 0.75 mm (the middle of the carrier’s 0.5-1 mm): time to cross = 0.75 ÷ 2.8 = 0.27 s.
- Compare 15 s without discs: the stack is 15 ÷ 0.27 = about 56 times faster at the same g.
Worked Example 2: The Settling Area
A standard measure of a disc machine is its equivalent settling area, Σ = 2π × N × ω² × (R³ − r³) ÷ (3 × g × tan β), where N is the number of discs, R and r the outer and inner radii of the stack and β the cone angle. This formula is recalled from the separation literature, not from the wiki. Use N = 30 (the carrier gives 20-40), R = 60 mm, r = 20 mm, β = 40°, and ω² = 701,838 (8,000 RPM).
- R³ − r³ = 0.060³ − 0.020³ = 2.16 × 10⁻⁴ − 8 × 10⁻⁶ = 2.08 × 10⁻⁴ m³.
- Numerator: 2π × 30 × 701,838 × 2.08 × 10⁻⁴ = 27,520.
- Denominator: 3 × 9.81 × tan 40° = 3 × 9.81 × 0.839 = 24.7.
- Σ = 27,520 ÷ 24.7 = 1,114 m².
The ideal capacity for removing globules of a given size is about 2 × v_g × Σ, where v_g is the 1 g rise speed, 6.7 × 10⁻⁷ m/s for the 4 µm globule.
- Q = 2 × 6.7 × 10⁻⁷ × 1,114 = 1.5 × 10⁻³ m³/s = 5.4 m³/h, that is 5,400 L/h.
- For a 1.5 µm globule, the rise speed falls by (1.5 ÷ 4)² = 0.14, so Q = about 750 L/h.
Both are idealized upper bounds that real machines meet only in part, because flow in the channels is not perfectly even. Even so, they are about 7 to 54 times the 100 L/h upper end of the carrier’s 50-100 L/h for a hand separator. The conclusion is a design priority: for a small hand-powered separator the limit is the power available (see Continuous Flow System) and the accuracy of the gaps, not the stack’s settling area.
Gap Accuracy
Flow between two flat plates carries a rate that goes as the cube of the gap, so small errors in spacing become large errors in flow.
- A gap of 0.85 mm instead of 0.75 mm: (0.85 ÷ 0.75)³ = 1.46, so that channel carries 46% more milk than the nominal.
- A gap of 0.65 mm: (0.65 ÷ 0.75)³ = 0.65, so it carries 35% less.
Channels with too much flow let fat through into the skim milk; channels with too little waste capacity. A tolerance of ±0.1 mm on 0.75 mm therefore swings the flow in the channels by about 1.5 times up and down. The spacer dimples in the carrier’s design must be even, and the discs flat.
What You Need
- Stainless steel sheet, 0.5 mm, for stamping the conical discs with spacer dimples; see Stainless Steel Production
- A stamping die or a spinning (metal-forming) lathe for the cones
- A shaft or rod with a keyway or notches, so the distribution holes line up in every disc
Method: Stack And Fit
Step 1: Form identical discs. A burr or crease alters the gap.
Step 2: Put an aligned ring of holes in each, so the holes form a vertical channel that carries the milk to the stack.
Step 3: Stack them on a keyed center tube in one order, with the dimples facing the same way.
Step 4: Lock the stack with the top nut. The thread direction is chosen so that the bowl accelerating tightens the nut rather than loosening it; check with a slow turn before a first run.
Step 5: Weigh and balance the assembled bowl; see Dynamic Balancing.
Common Mistakes
- Bent discs. One bent disc ruins the gaps around it.
- Mixing discs from different stacks. The holes no longer align.
- Too many discs. Add discs only if you can hold the gap and the weight.
- Not washing between runs. Milk dries between discs; see Cleaning and Sanitizing Without Chemicals.
Safety
A stack that comes loose at speed is a rotor failure
A loose top nut or a disc stack that shifts unbalances a bowl that turns at 62.8 m/s at its rim; see Speed Limits and Material Stress. Never run a separator with a missing disc, a cracked disc or a nut you cannot confirm is tight, and stop at the first sign of vibration.
What’s Next
- Learn next: Bowl Construction — the shell that holds the stack
- Combine with: Cream Separation Techniques — the dairy view